Method and apparatuses for ion cyclotron spectrometry

ABSTRACT

An ion cyclotron spectrometer may include a vacuum chamber that extends at least along a z-axis and means for producing a magnetic field within the vacuum chamber so that a magnetic field vector is generally parallel to the z-axis. The ion cyclotron spectrometer may also include means for producing a trapping electric field within the vacuum chamber. The trapping electric field may comprise a field potential that, when taken in cross-section along the z-axis, includes at least one section that is concave down and at least one section that is concave up so that ions traversing the field potential experience a net magnetron effect on a cyclotron frequency of the ions that is substantially equal to zero. Other apparatuses and a method for performing ion cyclotron spectrometry are also disclosed herein.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/833,079, filed Aug. 2, 2007, now U.S. Pat. No. 7,777,182, issued Aug.17, 2010, the disclosure of which is hereby incorporated herein by thisreference in its entirety.

GOVERNMENT RIGHTS

This invention was made with government support under Contract No.DE-AC07-051D14517 awarded by the United States Department of Energy. Thegovernment has certain rights in the invention.

TECHNICAL FIELD

This invention relates to mass spectrometry in general and, morespecifically, to ion cyclotron mass spectrometry.

BACKGROUND

Ion cyclotron resonance mass spectrometry (ICR-MS) involves excitingions at their ion cyclotron resonance (ICR) frequency and then observingthe transient decay of the image currents induced on detection plateslocated adjacent the resonating ions. The electrical signals from thedetection plates may be Fourier-transformed to produce frequency or massspectrum data. Ion cyclotron resonance mass spectrometry that involvessuch Fourier transformations may also be referred to asFourier-transform ion cyclotron resonance mass spectrometry,“FT-ICR-MS,” “FTICR,” or simply “FTMS.”

Fourier transform ion cyclotron resonance mass spectrometry differs fromother mass spectrometry techniques in that the ions are not detected byhitting a detector, but only by passing near detection plates.Additionally, the ion masses are not resolved in space or in time aswith other techniques, but only in frequency. Stated another way, thedifferent ions are not detected in different places as with sectorinstruments or at different times as with time-of-flight instruments.

A typical FTICR spectrometer involves a cube-shaped container or “cell”having three opposed sets of plates arranged to form the cube shape. Theplates comprising the cell are positioned in a uniform magnetic field sothat one pair of plates is orthogonal to the magnetic field, whereas theother two pairs of plates are generally parallel to the magnetic field.The three sets of plates are electrically connected to a voltage sourcethat is operable to place various voltages on the plates to achievevarious operational modes for the cell. The pair of plates that isorthogonal to the magnetic field is often referred to as the trappingplates. One of the other sets of opposed plates, commonly referred to asthe excitation plates, is used to excite the ions, whereas the otheropposed set of plates, commonly referred to as the detection plates, isused to detect the resonating ions.

In operation, a trapping voltage is placed on the trapping plates,creating an electric field within the cell. The combination of theelectric and magnetic fields confine or trap the ions within the cell.Generally speaking, the ions are radially confined by the magnetic fieldand axially confined by the electric field. The ions oscillate betweenthe trapping plates at a so-called z-axis trapping frequency, as theCartesian z-axis is often selected as the axis that passes through thetrapping plates. The electric and magnetic fields induce three generalmotions on the ions trapped within the cell: cyclotron motion, trappingmotion, and magnetron motion. The magnetic field causes the ions to havea cyclotron frequency that is inversely proportional to theirmass-to-charge (m/z) ratio. That is, ions with smaller m/z ratios willhave higher cyclotron frequencies, whereas those with larger m/z ratioswill have lower cyclotron frequencies. The ions trapped within the cellmay be produced within the cell itself (e.g., by any of a wide range ofionization techniques), or may be produced external to the cell, thenintroduced into the cell by appropriate means.

Once a suitable number of ions is trapped within the cell, the cell maybe switched to an excitation mode of operation, in which the ions areexcited while remaining trapped within the cell. In the excitation modeof operation, an excitation signal (e.g., an alternating signal in theradio-frequency (RF) range) is applied on the two excitation plates.Generally speaking, the RF signals applied to the opposing excitationplates are 180° out of phase relative to each other. Because theexcitation electric field is applied in addition to the trapping field,the ions will remain trapped within the cell (i.e., the ions are stillsubject to the trapping field), even as they are excited or energizeddue to the application of the excitation electric field. The RFexcitation signal can be applied as a discrete frequency, as multiplediscrete frequencies, or as a “chirp” in which the frequency is sweptthrough a defined range of amplitudes and frequencies. Just as a tuningfork of the same frequency can gain energy from a similar tuning forkvibrating in a room, the ions trapped within the cell gain energy whenthe frequency of the voltage on the excitation plates is the same as theion resonance cyclotron frequency. The increased energy of the ionscauses the radii of the ion orbits to increase while the cyclotronfrequency of the ions remains the same.

After the excitation event, the resonating ions of equivalentmass-to-charge (m/z) ratio are substantially in phase (i.e., coherent)and at sufficiently large orbits (i.e., their cyclotron radii haveincreased even though their cyclotron frequencies remain the same) thatthey can be detected by the detection plates. If the ions are positivelycharged, they attract electrons in the detection plates as they pass by,thus inducing a signal. If the ions are negatively charged, they repelthe electrons in the detection plates. The signal between the twoopposed detection plates has the same frequency as the cyclotronfrequency of the resonating ions. The signal may then be amplified,digitized, and Fourier-transformed into mass spectrum data.

More recently, FTICR mass spectrometers have been developed wherein thecell takes the form of a Penning trap. Briefly, a Penning trap is adevice that utilizes a linear magnetic field and a quadrupole electricfield to confine ions or other charged particles within the trap. Thequadrupole electric field may be generated by using a set of threeelectrodes, a ring and two end cap electrodes, for example. The ring andend cap electrodes are typically hyperboloids of revolution so that theelectric field created between the electrodes has the desired quadrupoleshape.

In theory, the effective cyclotron frequency of an ion trapped withinthe FTICR mass spectrometer will be independent of the position of theion within the spectrometer as well as its cyclotron radius. Accordingto the literature, the desired field within an FTICR is a quadrupolefield. Unfortunately, however, this effect is only approached near thecenter or “saddle” of the quadrupole field. Ions located away from thecenter or saddle of the field are subject to various non-linearitiesthat adversely affect their behaviors and the resulting data. Forexample, the trapping electric field produced by the end cap electrodestends to shield or insulate ions located near the end cap electrodesfrom the excitation field. Consequently, ions located near the end capelectrodes are not excited to the same degree as ions located elsewhere.The shielding effect also makes it more difficult to detect excited ionsnear the end cap electrodes. The problem is made worse by the fact thatthe ions spend a substantial portion of time near the end capelectrodes, i.e., where the excitation and detection processes are leastefficient.

Still other problems arise from the fact that most of the quadrupolefield lines are not parallel to the magnetic field lines. Thenon-parallel fields induce magnetron effects on the ions, which aredetrimental. For example, induced magnetron effects in cells utilizing aquadrupole field decrease the cyclotron frequency of the ions. While itis possible to correct the reduced cyclotron frequency (e.g., viacalibration), the induced magnetron effects also lead to a loss of ioncloud coherence, which results in reduced resolution. An excessivenumber of ions are also lost to the cell in such systems, which limitsresolution and detection limits. The induced magnetron effects, as wellas non-linearities in the excitation and detection of the ions, alsolimits the effective mass-to-charge ratio range that can be observed fora given set of operational parameters. Consequently, Fourier transformion cyclotron resonance spectroscopy systems have failed to realizetheir full potential.

SUMMARY OF THE INVENTION

Ion cyclotron spectrometry apparatus may include a vacuum chamber thatextends at least along a z-axis. A plurality of opposed electrode pairsare positioned within the vacuum chamber so that they extend along thez-axis. The apparatus may also include means for producing a magneticfield within the vacuum chamber and between the plurality of opposedelectrode pairs so that a magnetic field vector between the plurality ofopposed electrode pairs is generally parallel to the z-axis. A voltagesource electrically connected to the plurality of opposed electrodepairs applies a voltage function to the plurality of opposed electrodepairs so as to cause a substantially uniform electric field to beestablished between the plurality of opposed electrode pairs, thesubstantially uniform electric field including a plurality of potentiallines that are substantially parallel in the region between the opposedelectrode pairs.

Also disclosed is an electrode module that includes a generallyring-shaped field termination unit that defines an interior regiontherein. A first electrode is mounted within the interior region of thefield termination unit. A second electrode is also mounted within theinterior region of the field termination unit so that the first andsecond electrodes are positioned in generally parallel, spaced-apartrelation. The combination of the field termination unit and the firstand second electrodes is such that a voltage potential placed betweenthe first and second electrodes will result in the formation of anelectric field having potential lines that are substantially parallelthroughout a region defined between the first and second electrodes.

Another embodiment of ion cyclotron spectrometry apparatus may include avacuum chamber that extends at least along a z-axis. The apparatus mayalso include a magnetic field within the vacuum chamber so that amagnetic field vector is generally parallel to the z-axis. The apparatusis also provided with means for producing a trapping electric fieldwithin the vacuum chamber. The trapping electric field includes a firstsection that induces a first magnetron effect that increases thecyclotron frequency of an ion and a second section that induces a secondmagnetron effect that decreases the cyclotron frequency of the ion. Thecyclotron frequency changes induced by the first and second magnetroneffects substantially cancel one another so that the ion traversing thefirst and second sections will experience no net change in cyclotronfrequency.

Yet another embodiment of an ion cyclotron spectrometer may include avacuum chamber that extends at least along a z-axis. A plurality ofopposed electrode pairs are positioned within the vacuum chamber so thatthey extend along the z-axis. The apparatus may also include means forproducing a magnetic field within the vacuum chamber and between theplurality of opposed electrode pairs so that a magnetic field vectorbetween the plurality of opposed electrode pairs is generally parallelto the z-axis. A voltage source electrically connected to at least someof the plurality of opposed electrode pairs applies at least a trappingvoltage function to the plurality of opposed electrode pairs. Thetrapping voltage function results in the establishment of a trappingelectric field between the plurality of opposed electrode pairs. Thetrapping electric field includes at least a first section that induces afirst magnetron effect that increases a cyclotron frequency of an ionand at least a second section that induces a second magnetron effectthat decreases the cyclotron frequency of the ion so that the iontraversing the first and second sections will experience no net changein cyclotron frequency.

Also disclosed is a method for performing ion cyclotron spectrometrythat includes: providing ions within a vacuum chamber; producing amagnetic field within the vacuum chamber so that a magnetic field vectoris generally parallel to a z-axis of the vacuum chamber; producing atrapping electric field within the vacuum chamber, the trapping electricfield including at least a first section that induces a first magnetroneffect that increases a cyclotron frequency of an ion and at least asecond section that induces a second magnetron effect that decreases thecyclotron frequency of the ion so that the ion traversing the first andsecond sections will experience no net change in cyclotron frequency;exciting ions trapped by the magnetic and electric fields; and detectingexcited ions.

Still another embodiment of an ion cyclotron spectrometer may include avacuum chamber that extends at least along a z-axis. A plurality ofopposed electrode pairs are positioned within the vacuum chamber so thatthey extend along the z-axis. The apparatus may also include means forproducing a magnetic field within the vacuum chamber and between theplurality of opposed electrode pairs so that a magnetic field vectorbetween the plurality of opposed electrode pairs is generally parallelto the z-axis. A voltage source electrically connected to at least someof the plurality of opposed electrode pairs applies at least a trappingvoltage function to at least some of the plurality of opposed electrodepairs. The trapping voltage function results in the establishment of atrapping electric field between the plurality of opposed electrodepairs. The trapping electric field includes a field potential that, whentaken in cross-section along the z-axis, includes at least one sectionhaving a concave curvature (i.e., electrostatic refractive field) and atleast one section having a convex curvature so that ions traversing thefield potential sections having the concave and convex curvaturesexperience a net magnetron effect on the cyclotron frequency that issubstantially equal to zero.

Still yet another embodiment of an ion cyclotron spectrometer mayinclude a vacuum chamber that extends at least along a z-axis and meansfor producing a magnetic field within the vacuum chamber so that amagnetic field vector is generally parallel to the z-axis. Thespectrometer may also include means for producing a trapping electricfield within the vacuum chamber that includes a field potential that,when taken in cross-section along the z-axis, includes at least onesection that is concave down and at least one section that is concave upso that ions traversing the field potential sections experience a netmagnetron effect on the cyclotron frequency that is substantially equalto zero.

Another method for performing ion cyclotron spectrometry may include:providing ions within a vacuum chamber; producing a magnetic fieldwithin the vacuum chamber so that a magnetic field vector is generallyparallel to a z-axis of the vacuum chamber; producing a trappingelectric field within the vacuum chamber, the trapping electric fieldcomprising a field potential that, when taken in cross-section along thez-axis, includes at least one section that is concave down and at leastone section that is concave up so that ions traversing the fieldpotential sections experience a net magnetron effect on the cyclotronfrequency that is substantially equal to zero; exciting ions trapped bythe magnetic and electric fields; and detecting excited ions.

Another embodiment of an ion cyclotron spectrometer may include a vacuumchamber that extends at least along a z-axis, along with means forproducing a magnetic field within the vacuum chamber so that a magneticfield vector is generally parallel to the z-axis. The spectrometer mayalso include means for producing a trapping electric field within thevacuum chamber, the trapping electric field inducing compensatedmagnetron effects so that a net change in cyclotron frequency of an ionis substantially equal to zero.

In yet another embodiment, an ion cyclotron spectrometer may include avacuum chamber that extends at least along a z-axis, along with meansfor producing a magnetic field within the vacuum chamber so that amagnetic field vector is generally parallel to the z-axis. Thespectrometer may also include means for producing a trapping electricfield within the vacuum chamber, the trapping electric field inducingcompensated magnetron effects so that confined ions experience a netphase shift that is substantially equal to zero.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred embodiments of the invention areshown in the accompanying drawings in which:

FIG. 1 is a cross-sectional schematic view in elevation of oneembodiment of an ion cyclotron spectrometer;

FIG. 2 is a perspective view of a plurality of electrode modulesarranged in a stacked configuration to form a cell;

FIG. 3 is a perspective view of a single electrode module of the typeillustrated in FIG. 2;

FIG. 4 is a side view in elevation of a single electrode module of thetype illustrated in FIG. 2;

FIG. 5 a is a computer-generated representation of electric fieldpotential lines formed between an electrode pair with no static chargeresiding on the insulating support posts;

FIG. 5 b is a computer-generated representation of electric fieldpotential lines formed between an electrode pair with a static chargeresiding on an upper pair of insulating support posts, but no staticcharge residing on a lower pair of insulating support posts;

FIG. 6 a is a computer-generated representation of a trapping electricfield produced by a plurality of electrode pairs of a spectrometer;

FIG. 6 b is a computer-generated representation of an electric fieldpotential associated with the electric field of FIG. 6 a;

FIG. 7 is a computer-generated simulation of an excitation path followedby an ion located between an opposed electrode pair;

FIG. 8 is a computer-generated simulation in the x-z plane of excitationradii of ions with the same m/z ratio;

FIG. 9 is a computer-generated simulation in the x-z plane of excitationradii of ions with different m/z ratios;

FIGS. 10 a and 10 b are electrical schematics of one embodiment of acontroller for electrically connecting various electrode pairs to avoltage source and a detector; and

FIG. 11 is a computer-generated representation of a trapping electricfield produced by a cell illustrated in FIG. 10 b.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of an ion cyclotron spectrometer 10 according to theteachings of the present invention is best seen in FIGS. 1 and 2 and maycomprise a vacuum chamber 12 that is positioned within a magnetic fieldso that a magnetic field vector (represented by arrows 14) is generallyaligned with a longitudinal or z-axis of vacuum chamber 12. A pluralityof electrode pairs 16 are positioned within the vacuum chamber 12 sothat the electrode pairs 16 extend generally along the longitudinal orz-axis of vacuum chamber 12. The plurality of electrode pairs 16generally define a region or “cell” 18 within which ions 20 may beconfined, excited, and detected in accordance with a variety ofprocesses that will be described in further detail below. The variouselectrode pairs 16 that define cell 18 are electrically connected to avoltage source 22, such as, for example, via controller 23. Voltagesource 22 may be operated to apply to the various electrode pairs 16several different types of voltage functions (e.g., DC, RF, andcombinations thereof) to allow the spectrometer 10 to be operated in thevarious operational modes (e.g., confinement, excitation, and detectionmodes) described herein.

Before proceeding with the description, it should be noted that asignificant aspect of the present invention involves the recognitionthat substantial improvements in ion cyclotron resonance spectroscopycould be realized if an ion cyclotron resonance cell could be made tosimulate, as nearly as possible, two infinite parallel plates. A cellsimulating such a configuration would be capable of linear excitationand detection of ions, which would allow ion cyclotron resonancespectroscopy to actually realize all of the benefits and advantages thatare theoretically possible in such spectroscopy. That is, in a cell thatsimulates such a configuration, ions within the cell would beconsistently excited regardless of their locations between the plates.Likewise, the detection of the ions would be consistently uniform, againwithout regard to their locations between the plates. Of course, it isnot practical to use a set of infinite parallel plates, because theycannot be arranged within a magnetic field that is practical to achieve.Even if this problem is addressed, there must be some way to trap theions within a region of the cell without compromising the ideal lineargradient field.

The present invention recognizes that the need for infinite parallelplates can be dispensed with if the cell is provided with matchedlateral linear electric fields that extend in the same direction as themagnetic field. That is, because the magnetic field acts to limit themotions of the ions to cyclotron orbits in the x,y plane (assuming az-axis that is parallel to the magnetic field), an apparent (to theions) laterally infinite linear gradient field can be induced within acell that is not much longer than the distance between the plates.

In addition, the present invention recognizes that the design of thecell can be broken down into a DC problem and an AC problem. Theexcitation and detection of ions is an AC problem, in that they requiretime-varying electric fields. Ion trapping, on the other hand, is a DCproblem. That is, the electric field must have a potential “well” inorder to trap the ions between the plates so that they can be excitedand detected. The embodiments of the present invention solve theseproblems by slicing or dividing the cell into a plurality of opposedelectrode pairs 16 that extend along a magnetic field. Appropriate DCand AC voltages can then be provided to the opposed electrode pairs 16to provide for the trapping, excitation, and detection modes ofoperations described herein.

For example, when the spectrometer 10 is operated in a confinement ortrapping mode, voltage source 22 is operated so as to apply to thevarious electrode pairs 16 a trapping voltage function, which maycomprise a time-invariant or DC voltage. The application of the trappingvoltage function to the various electrode pairs 16 results in thecreation of a trapping electric field 24 (FIG. 6 a) within the cell 18,i.e., in the region located between the various opposed electrode pairs16. The trapping electric field 24, in combination with the magneticfield, functions to contain or trap ions 20 within the cell 18 ofspectrometer 10.

Referring now primarily to FIGS. 6 a and 6 b, one embodiment of atrapping electric field 24 (FIG. 6 a) comprises a field potential 26(FIG. 6 b) that, when taken in cross-section along the z-axis of thevacuum chamber 12 (FIG. 1), includes at least one convex section 28having a convex “curvature” (i.e., electrostatic refractive field) andat least one concave section 30 having a concave “curvature.” In theembodiment shown and described herein, the convex section 28 and concavesection 30 are located between two side portions or “wings” 27 and 29.As will be described in greater detail below, ions 20 oscillatingbetween the wings 27 and 29 and traversing the convex section 28 andconcave section 30 of trapping electric field 24 experience a netmagnetron effect on the cyclotron frequency that is substantially equalto zero. That is, any magnetron effect on the cyclotron frequency thatis induced in the ions 20 as they move in trapping electric field 24 iscompensated by the configuration of the trapping electric field 24 andresulting field potential 26.

With specific reference now to the embodiment illustrated in FIGS. 6 aand 6 b, the field potential 26 of trapping electric field 24 comprisestwo convex sections, i.e., first and second convex sections 28 and 28′,between which is located the concave section 30. The magnetron effectcompensation occurs as follows: as a result of the forces exerted onions 20 by the trapping electric field 24 and the magnetic field, ions20 will generally follow reciprocating paths (i.e., along the z-axis)between the wings 27 and 29. That is, the ions 20 will travel back andforth between the wings 27 and 29, traversing the convex section 28 andconcave section 30 as they do so. The magnetron effect induced by theconvex sections 28, 28′ leads to an increase in the cyclotron frequency,whereas the magnetron effect induced by the concave section 30 leads toa decrease in the cyclotron frequency. Thus, the combined magnetroneffects are such that the changes in cyclotron frequency substantiallycancel each other. Consequently, the ion 20 experiences a net magnetroneffect that is zero (or substantially equal to zero) each time it fullycrosses or traverses the convex section 28 and concave section 30 of thefield potential 26 of trapping electric field 24. Hence, it is possibleto detect the “true” cyclotron frequency (i.e., “true” meaningsubstantially equal to the actual cyclotron frequency) of the ions for agiven magnetic field strength.

Another way to view the induced magnetron effects is that theelectrostatic component in the convex section 28 of field potential 26results in an increase in cyclotron frequency. On the other hand, theelectrostatic component in the concave section 30 results in a decreasein cyclotron frequency. If the electrostatic components of the convexsection 28 and concave section 30 are matched (or nearly so), theoverall magnetron effect will be reduced or eliminated (i.e., themagnetron effect will be compensated), with the effect that the ions 20will experience substantially no net change in cyclotron frequency.Stated another way, the trapping electric field 24 includes at least afirst convex section 28 that induces a first magnetron effect thatincreases a cyclotron frequency of an ion 20 and at least a secondconcave section 30 that induces a second magnetron effect that decreasesthe cyclotron frequency of an ion 20, so that an ion 20 traversing thefirst convex section 28 and the at least a second concave section 30will experience substantially no net change in cyclotron frequency.Thus, the observed cyclotron frequency will be the true or substantiallyequal to the true cyclotron frequency of the ions 20 in a magnetic fieldof a given strength.

As will be described in greater detail below, any of a wide range oftrapping electric fields may be utilized in the present invention solong as the configuration of the electric field is such that themagnetron effect is compensated, i.e., so that ions 20 experience no netchange in cyclotron frequency. One type of trapping electric field 24that is effective in compensating for magnetron effect (i.e., whereinthe net magnetron effect induced on ions 20 traversing the trappingelectric field 24 is substantially equal to zero) is a trapping electricfield 24 that has a field potential 26 that follows the n^(th) rootpower law. By way of example, in the embodiment shown and describedherein, the field potential 26 of trapping electric field 24substantially follows a curvature described by a second root power law(e.g., 10^(1/2)). In another embodiment, the field potential 26 oftrapping electric field 24 follows a curvature described by a third rootpower law (e.g., 10^(1/3)).

The spectrometer 10 may also comprise a detector 36 (as shown in FIG. 1)that is electrically connected to at least some of the opposed electrodepairs 16. Detector 36 detects electrical signals 38 induced in thevarious opposed electrode pairs 16 by resonating ions 20 trapped withincell 18. Detector 36 produces output signals 42 that are related to theresonating ions 20. A data processor 40 operatively associated withdetector 36 may process output signals 42 produced by detector 36 toproduce processed data 44. For example, in one embodiment, dataprocessor 40 may be provided with a Fourier transform algorithm suitablefor performing a Fourier transform on output signals 42 from thedetector 36. Accordingly, the processed data 44 will compriseFourier-transformed data. Processed data 44 from data processor 40 maythereafter be presented in a suitable display 46 or any other device orsystem that will allow a user to interpret the processed data 44.

The ion cyclotron spectrometer 10 may be operated as follows to performFourier transform ion cyclotron resonance spectroscopy. Alternatively,other types of ion cyclotron spectrometry could be performed, as will bedescribed in further detail herein. As a first step, vacuum chamber 12may be positioned within a magnetic field so that the magnetic fieldvector 14 is generally parallel with the z-axis of vacuum chamber 12.See FIG. 1. The magnetic field should be of sufficient strength so that,when combined with the trapping electric field 24, ions 20 will beconfined or trapped within the cell 18. By way of example, in oneembodiment, a magnetic field having a strength of about 7 tesla in thecell 18 will provide acceptable results, although magnetic fields havingother strengths (i.e., higher or lower) may also be used. Vacuum chamber12 should also be evacuated and provided with ions 20 to be studied.

The ions 20 to be studied may be produced in accordance with any of awide variety of processes that are now known in the art or that may bedeveloped in the future. For example, in one embodiment, the ions 20 areproduced by a suitable ionizer (not shown) located outside the cell 18,then conducted into the cell 18 by an ion “gate” or guide.Alternatively, ions 20 may be produced within the cell 18 itself, suchas, for example, by ionizing sample material previously provided withincell 18. In any event, once ions 20 have been introduced into the cell18, voltage source 22 is operated to place a trapping voltage functionon at least some of the various opposed electrode pairs 16. As a result,the trapping electric field 24 (for example, an electric field having afield potential in accordance with the n^(th) root power law)illustrated in FIG. 6 a will be created in the cell 18.

As may be appreciated with reference to FIG. 6 b, ions 20 confined bythe trapping electric field 24 will travel back and forth (e.g., alongthe z-axis) between wings 27 and 29 of field potential 26 of cell 18,repeatedly traversing the convex sections 28, 28′ and concave section 30as they do so. The magnetron effect induced in the ions 20 will becompensated with each traversal of the convex sections 28, 28′ andconcave section 30.

At the appropriate time, the voltage source 22 may be operated in anexcitation mode in order to excite the ions 20 contained within the cell18. In operating in the excitation mode, voltage source 22 places anexcitation voltage function on at least some of the opposed pairs ofelectrodes 16. It should be noted that the excitation voltage functionis provided in addition to the trapping voltage function so that ions 20can be excited but still remain trapped within the cell 18. Generallyspeaking, the excitation voltage function will cause an alternatingelectric field to be established in the region between the opposed pairsof electrodes 16 that will be effective in causing ions 20 within thecell 18 to become excited (i.e., to gain energy).

During the excitation mode of operation, ions 20 within cell 18 willcontinue to move generally axially back and forth (i.e., in areciprocating manner) within cell 18, with their magnetron effects beingcompensated each time the ions 20 traverse the convex sections 28, 28′and concave section 30 of field potential 26 (FIG. 6 b). In addition tothe generally axial or reciprocating motion followed by ions 20 withincell 18, the orbits or cyclotron radii of the ions 20 will increase asthe ions 20 continue to absorb energy from the alternating (e.g., RF)excitation electric field.

As will be described in further detail herein, the excitation voltagefunction may comprise any of a wide range of functions suitable forexciting the ions 20 of interest. For example, in one embodiment, theexcitation voltage function may be selected to cause the alternatingelectric field to vary at a fixed frequency or at multiple fixedfrequencies. Alternatively, the excitation voltage function may causethe alternating electric field to vary across a predetermined frequencyrange, also known as a “chirp” function.

When the ions 20 are exposed to the excitation electric field, theyresonate, continually gaining energy, which results in an increase inthe cyclotron radius of the ions 20. The increase in the cyclotronradius is schematically illustrated in FIG. 7, which is a computersimulation that depicts a spiral path 48 followed by a resonating ion 20trapped within cell 18 (see FIG. 1). As the ions 20 resonate within thecell 18, they also follow reciprocating axial paths (i.e., generallyback and forth along the z-axis) within vacuum chamber 12. As a result,the resonating ions 20 repeatedly traverse the convex sections 28, 28′and concave section 30 of the field potential 26 (FIG. 6 b).

Because the trapping electric field 24 results in a no-net magnetroneffect as resonating ions 20 repeatedly traverse the convex sections 28,28′ and concave section 30, the net cyclotron frequency of theresonating ions 20 remains unchanged. Computer simulations confirm thisfact as indicated by the straight vertical line 50 on the spiral path 48for an exemplary resonating ion 20. More specifically, and as will bedescribed in greater detail below, the line 50 connects the locus ofpoints that correspond to the position of the exemplary resonating ion20 at intervals of the time period for the cyclotron frequency of theion 20. Because the points occur at the same angular position on thespiral path 48, i.e., because line 50 is substantially straight, theresonating ion 20 is revolving at the true cyclotron frequency for theion 20.

After a suitable period of excitation, ions 20 trapped within the cell18 may be detected by detector 36 (e.g., via controller 23, as will bedescribed in greater detail below). In one embodiment, detector 36detects electrical signals 38 induced on the various opposed pairs ofelectrodes 16 by the movement of the resonating ions 20. The dataprocessor 40 may be used to process output signals 42 from detector 36in accordance with a transform algorithm to produce processed data 44.By way of example, in an embodiment wherein the data processor 40 isprovided with a Fourier transform algorithm, processed data 44 maycomprise Fourier-transformed data. The Fourier-transformed data may thenbe presented on display 46.

A significant advantage of the spectrometer 10 according to the presentinvention relates to the trapping electric field 24. More specifically,the trapping electric field 24 effectively compensates for magnetroneffects induced in the ions 20 as they move within cell 18. That is,ions 20 traversing the trapping electric field 24 experience a netmagnetron effect on the cyclotron frequency that is substantially zero.Many advantages and benefits flow from the compensated magnetroneffects.

For example, because the net magnetron effect is substantially zero, theions 20 will resonate at their true (or substantially equal to theirtrue) cyclotron frequency, thereby simplifying the excitation anddetection of the resonating ions 20. Moreover, because the observedfrequency of the ions 20 is substantially identical to the truecyclotron frequency, the calibration process for the Fourier transformmass spectrometry process is greatly simplified. That is, there is noneed to take into account a reduced cyclotron frequency resulting frommagnetron effects induced by the trapping electric field 24.

Another benefit of the present invention is that all ions 20 of the sametype will be excited to the same radius throughout the cell 18, as bestseen in FIG. 8. Moreover, ions 20 of different types will also all beexcited to the same radii, as best seen in FIG. 9. Consequently, theoutput signals 42 (e.g., amplitude or area of the peaks) from detector36 will more accurately reflect the actual number of ions 20 within thecell 18, thereby allowing more accurate isotope ratio and otherquantitative measurements to be made. In addition, the ion “clouds” willremain coherent for a longer period of time (relative to conventionalion cyclotron resonance spectroscopy systems), thereby improvingresolution and mass accuracy.

Yet another benefit associated with the field potential 26 of thetrapping electric field 24 is that the ions 20 travel faster (along thez-axis) through the convex sections 28, 28′ and concave section 30 thanthrough the side portions or “wings” 27 and 29 of field potential 26(FIG. 6 b). Accordingly, the ions 20 spend substantially more time ineither of the side portions 82 and 84 than the middle portion 86 of thetrapping electric field 24, where the cyclotron frequency is mostconsistent (FIG. 6 a).

Still other advantages are associated with the overall configuration ofthe opposed pairs of electrodes 16 and the fact that no end-capelectrodes are required. For example, the electrode configuration of thespectrometer 10 is highly effective in simulating a cell defined byinfinite parallel electrodes. As a result, the excitation and detectionprocesses are substantially linear. That is, there are no end capelectrodes to shield the ions 20 from the excitation field and toprevent the ions 20 from being easily detected (due to the presence ofthe trapping electric field 24 emanating from the end cap electrodes).Consequently, a spectrometer 10 according to the present invention willrealize increased sensitivity and lower detection limits compared toconventional systems.

Still other advantages derive from the parallel plate design of thepresent invention. For example, the ions 20 tend to be more evenlydistributed along the length of the cell 18, which may reducespace-charge effects. In addition, the “open” configuration provided bythe opposed pairs of electrodes 16 also provides convenient pathways forintroducing ions into the cell 18 and allows for improved vacuumpumping.

Having briefly described one embodiment of an ion cyclotron resonancemass spectrometer 10 according to the present invention, as well as someof its more significant features and advantages, various exemplaryembodiments of the spectrometer 10 and methods for performing massspectrometry will now be described in detail.

Referring back now to FIGS. 1 and 2 simultaneously, one embodiment of anion cyclotron resonance spectrometer 10 may comprise a vacuum chamber 12that is configured to receive or contain the various components anddevices described herein. In one embodiment, vacuum chamber 12 is alsoconfigured to be positioned within a magnetic field so that a magneticfield vector 14 is generally parallel to the longitudinal or z-axis ofthe vacuum chamber 12. In this regard, it should be noted that it is notthe particular orientation per se of the vacuum chamber 12 with respectto the magnetic field that is important, but rather the orientations(with respect to the magnetic field vector 14) of the opposed pairs ofelectrodes 16 provided within the vacuum chamber 12. That is, theplurality of electrode pairs 16 should be positioned within the vacuumchamber 12 and the vacuum chamber 12 positioned within the magneticfield, so that plurality of opposed electrode pairs 16 extend generallyalong magnetic field vector 14, as best seen in FIG. 1.

The plurality of electrode pairs 16 generally define a region or cell 18within which ions 20 are confined, excited, and detected in order toperform, for example, Fourier-transform ion cyclotron resonance massspectrometry (FTICR-MS). Alternatively, other processes may be performedusing the components and devices shown and described herein.

In one embodiment, vacuum chamber 12 may comprise a generally elongate,cylindrically shaped structure that extends along the z-axis of anorthogonal x,y,z coordinate system, as best seen in FIG. 1.Alternatively, vacuum chamber 12 may comprise other shapes andconfigurations. In addition to containing the various opposed pairs ofelectrodes 16, vacuum chamber 12 may also be configured to operate orinterface with any of a wide variety of ancillary components and devices(not shown), such as vacuum pumps, pressure sensors, ionizationchambers, and the like, that may be required or desired in anyparticular application. In addition, vacuum chamber 12 may also beprovided with one or more access ports 52 or “feed-throughs” to allowvarious external devices and systems, such as, for example, voltagesource 22 and detector 36, to be operably connected to the appropriatecomponents (e.g., the opposed electrode pairs 16) housed within vacuumchamber 12. Vacuum chamber 12 may also be configured to be operativelyassociated with a separate ionization device (not shown) to allow ions20 from the ionization device to be conducted to the cell 18.

However, because vacuum chambers, as well as the various ancillarycomponents and devices that may be required or desired for performingion cyclotron resonance spectroscopy, are well known in the art andcould be readily provided by persons having ordinary skill in the artafter having become familiar with the teachings provided herein, theparticular vacuum chamber 12 and various ancillary components anddevices that may be utilized in one embodiment of the invention will notbe described in further detail herein.

As already mentioned, vacuum chamber 12 may be configured to bepositioned within a magnetic field so that the magnetic field vector(illustrated by arrows 14 in FIG. 1) is generally parallel to thelongitudinal or z-axis of the vacuum chamber 12. The magnetic field maybe produced by a suitable magnet, such as a superconducting magnet (notshown), located outside vacuum chamber 12. If an external magnet isused, vacuum chamber 12 should be constructed from a non-magneticmaterial (e.g., non-magnetic stainless steel) so as to avoid diminishingor perturbing the strength of the magnetic field within vacuum chamber12. Alternatively, other arrangements are possible, as would becomeapparent to persons having ordinary skill in the art after having becomefamiliar with the teachings provided herein. Consequently, the presentinvention should not be regarded as limited to any particulararrangement for the vacuum chamber 12 or for the production of themagnetic field within cell 18.

Before proceeding with the description, it should be noted that themagnetic field should be made as uniform as possible within the regiondefined by the cell 18, as non-uniformities in the magnetic field mayreduce the coherence of the excited ions 20. In addition, the strengthof the magnetic field should be commensurate with the strengthsgenerally preferred in ion cyclotron resonance spectrometry and suitablefor confining ions 20 when used in conjunction with the strength of thetrapping electric field 24 (FIG. 6 a) contained within the cell 18.Consequently, the present invention should not be regarded as limited toa magnetic field having any particular strength or range of strengths.However, by way of example, in one embodiment, the magnetic field withinthe vacuum chamber 12 has a strength of about 7 tesla.

Referring now to FIGS. 1-4, each opposed pair of electrodes 16 maycomprise an electrode assembly or module 54, a plurality of which may beassembled or “stacked” in side-by-side relationship to form the cell 18,as best seen in FIGS. 1 and 2. Each electrode assembly or module 54 maycomprise a generally ring-shaped field termination unit 56 within whichare mounted a pair of opposed electrodes 58 and 60. Alternatively, thefield termination units 56 need not be ring shaped but could compriseother shapes and configurations as well. Consequently, the presentinvention should not be regarded as limited to field termination units56 having the ring shape illustrated herein. Each electrode 58, 60 maybe mounted to the field termination unit 56 by a pair of insulatingsupport posts 62 so that the electrodes 58 and 60 are held in generallyparallel, spaced-apart relation in the manner best seen in FIGS. 3 and4. In the embodiment shown and described herein, the insulating supportposts 62 are hollow and are sized to receive screws 63 that are used tofasten or secure the electrodes 58, 60 to the field termination units56. The screws 63 may also provide a convenient means to electricallyconnect the electrodes 58, 60 to the voltage source 22 and detector 36,although other arrangements are possible. Each electrode 58 and 60 maybe provided with a pair of inwardly turned ends 64, 66, respectively.Field termination unit 56 may also be provided with a pair of inwardlyprojecting fins 68 located at positions about midway between theelectrodes 58 and 60, as best seen in FIGS. 3 and 4.

In addition to providing a convenient mounting arrangement for theelectrodes 58 and 60, certain structural features of the electrodemodule 54 and electrodes 58 and 60 aid in the creation and establishmentof a highly uniform electric field (e.g., trapping electric field 24) inthe region between the two electrodes 58 and 60. See FIGS. 5 a and 5 b.More specifically, and as will be described in greater detail below, thecombination of the ring-shaped field termination unit 56, the fins 68,and the inwardly turned ends 64, 66 of electrodes 58, 60 helps to“terminate” the trapping electric field 24 in the regions near the endsof the electrodes 58, 60 so that potential lines 61 of trapping electricfield 24 are substantially parallel in the entire region between theelectrodes 58 and 60. Consequently, the linear electrostatic AC fields“appear” to the ions 20 to be infinite.

The electrodes 58 and 60 provided within electrode module 54 may besubstantially identical to one another and, in one embodiment, maycomprise generally elongate, slender members as best seen in FIGS. 2-4.While the electrodes 58 and 60 may comprise a wide range of sizesdepending on the particular application, in one embodiment, eachelectrode 58 and 60 may have a length 70 in a range of about 50 mm toabout 90 mm (about 78 mm preferred), a width 72 in a range of about 3 mmto about 12 mm (about 7 mm preferred), and a thickness 74 in a range ofabout 0.5 mm to about 2 mm (about 1 mm preferred). The distance 78separating the opposed electrodes 58 and 60 may be in a range of about20 mm to about 50 mm (about 30 mm preferred).

It should be noted that the various dimensions for the electrodes 58, 60may be interrelated. That is, a change in one dimension may require achange in another dimension in order to result in satisfactory operationor to optimize performance. For example, in order to have the electrodes58 and 60 simulate as closely as possible infinite, parallel plates, thelength 70 of electrodes 58 and 60 should be at least twice that of thedistance 78 that separates them. In one embodiment, the length 70 ofelectrodes 58, 60 is about 2.6 times the distance 78 separating theelectrodes 58, 60. Of course, electrodes 58 and 60 having even longerlengths would be even more desirable. However, the maximum electrodelength 70 will usually be dictated by other factors, such as, forexample, the size of the vacuum chamber 12 and/or the size of the magnetused to produce the magnetic field.

The electrodes 58 and 60 may be fabricated from any of a wide range ofmaterials suitable for the particular application. By way of example, inone embodiment, the electrodes 58 and 60 are fabricated from “grade 2”titanium, although other materials, such as 316 stainless steel, couldalso be used. Similarly, the field termination unit 56 may be fabricatedfrom any of a wide range of materials suitable for the particularapplication. By way of example, in one embodiment, the field terminationunit 56 comprises an electrically conductive member and is manufacturedfrom grade 2 titanium. The insulating support posts 62 compriseelectrically insulating members and are manufactured from machinableglass ceramic material, such as MACOR®, available from CorningIncorporated, New York. Screws 63 are fabricated from titanium.Alternatively, other materials may be used for these components, aswould become apparent to persons having ordinary skill in the art afterhaving become familiar with the teachings provided herein.

As mentioned above, the electrode modules 54 are sized and configured sothat the trapping electric field 24 is substantially uniform in theregion between opposed electrodes 58 and 60, i.e., so that the potentiallines 61 of trapping electric field 24 are substantially parallel in theregion between the opposed electrodes 58 and 60, as best seen in FIGS. 5a and 5 b. A suitable design for the electrode modules 54 may bedeveloped with the aid of a computer program to model the trappingelectric field 24 for a given configuration of electrode module 54.Alternatively, other design methodologies and design tools may be used.By way of example, in one embodiment, the computer program may comprisea program commercially known as “SIMION® 7.0,” which is available fromScientific Instrument Services, Inc., 1027 Old York Road, Ringoes, N.J.08551 (USA). The trapping electric field 24 depicted in FIGS. 5 a and 5b was generated by the SIMION® 7.0 computer program based on anelectrode module 54 having the dimensions and configurations set forthherein.

FIG. 5 a depicts a trapping electric field 24 resulting from a voltageon electrode 58 of about +100 volts, a voltage on electrode 60 of −100volts, and a voltage on field termination unit 56 of about 0 volts. Thevoltage on insulating support posts 62 was also selected to be about 0volts. As can be seen in FIG. 5 a, the potential lines 61 of trappingelectric field 24 are highly uniform in the region between electrodes 58and 60, thereby indicating that the electrode module 54 is very good atsimulating an electric field that would be produced between twoinfinitely parallel plates.

FIG. 5 b depicts the trapping electric field 24 on the same electrodemodule 54, except that the upper insulating support posts 62 (i.e.,those supporting electrodes 58, 60) have acquired an electric charge ofabout 10 kilovolts (kV). As can be seen in the upper portion of FIG. 5b, the potential lines 61 of trapping electric field 24 are still highlyparallel in the region between electrodes 58 and 60, thereby indicatingthat the electrode module 54 is highly resistant to adverse effectsresulting from unwanted charge buildup on the insulating support posts62. That is, a charge buildup on insulating support posts 62 will nothave a significant effect on the operation of the spectrometer 10.

Referring back now to FIG. 2, the spectrometer 10 may comprise aplurality of electrode modules 54 mounted in side-by-side or “stacked”relationship in order to create a cell 18 having a desired number ofseparate, opposed pairs of electrodes 16. Each electrode module 54 maybe electrically insulated from an adjacent module by one or moreelectrically insulating spacer rings 80. Alternatively, other mountingarrangements may be utilized, as would become apparent to persons havingordinary skill in the art after having become familiar with theteachings provided herein. Insulating spacer rings 80 may be fabricatedfrom any of a wide range of materials suitable for the particularapplication. By way of example, in one embodiment, the insulating spacerrings 80 are fabricated from alumina. Alternatively, insulating spacerrings 80 could be fabricated from a machinable glass ceramic material,such as MACOR®. In one embodiment, a plurality of elongated rods (notshown) sized to be received by spacer rings 80 and field terminationunit 56 may be used to hold the various electrode modules 54 in thearrangements depicted herein. The elongated rods may be made from aninsulating material, such as alumina or MACOR®.

Generally speaking, it is desired, but not required, to position thevarious electrode modules 54 as close to one another as possible so asto minimize the likelihood that stray electric fields will penetrate thespace between adjacent electrode modules 54. By way of example, in oneembodiment, the electrode modules 54 are positioned so that they areseparated by a distance of about 3.8 mm. The spacing between any twoadjacent electrodes (e.g., between electrodes 58) is likewise about 3.8mm.

The various electrodes 58 and 60 of electrode modules 54 areelectrically connected to the voltage source 22, e.g., via screws 63, asbest seen in FIG. 1. Voltage source 22 is capable of providing variousvoltage potentials on the electrodes 58 and 60 so as to produce a widerange of static and dynamic electric fields in the manner describedherein. For example, the voltage source 22 may be used to provide atrapping voltage function on some or all of the various opposedelectrode pairs 16 in order to produce the trapping electric field 24illustrated in FIGS. 6 a and 6 b. Voltage source 22 may also be used toapply an excitation voltage function (e.g., typically in the form of analternating current of fixed or variable frequency) to some or all ofthe opposed electrode pairs 16 to produce an alternating electric field(not shown) suitable for exciting ions 20 contained within the cell 18.

Referring now primarily to FIGS. 6 a and 6 b, the trapping electricfield 24 (FIG. 6 a) utilized in one embodiment of the invention maycomprise a field potential 26 (FIG. 6 b) that, when taken incross-section along the z-axis of the vacuum chamber 12, includes atleast one convex section 28 having a convex curvature and at least oneconcave section 30 having a concave curvature. The trapping electricfield 24 and corresponding field potential 26 depicted in FIGS. 6 a and6 b were generated by the SIMION® computer program described above for acell configuration comprising twenty five (25) electrode pairs 16, asillustrated in FIG. 6 a. Alternatively, cell 18 could comprise adifferent number of electrode pairs 16. For example, another embodimentmay comprise thirteen (13) electrode pairs 16 (FIG. 11), as will bedescribed in greater detail below.

The trapping electric field 24 illustrated in FIG. 6 a comprises agenerally laterally symmetrical structure having first and second sideportions 82 and 84 connected together by a middle portion 86. The firstand second side portions 82 and 84 are located in the regions definedbetween twelve (12) opposed electrode pairs 16, whereas the middleportion 86 is located in the region defined between a single opposedelectrode pair 16. The configuration of field potential 26 of thetrapping electric field 24 is depicted in FIG. 6 b and substantiallyfollows the n^(th) root power law. Roughly speaking, the side portionsor wings 27 and 29 of field potential 26 are substantially linear andcorrespond to the first and second side portions 82 and 84 of trappingelectric field 24 (FIG. 6 a), whereas the convex section 28 and concavesection 30 of field potential 26 are non-linear and correspond to themiddle portion 86 of trapping electric field 24. The field potentials 26of the respective sections are substantially continuous, i.e., joinedtogether in a smooth manner. More specifically, the wings 27 and 29 offield potential 26 are joined together by first and second “convex”sections 28 and 28′, which are separated by “concave” section 30 andrespectively connected thereto by sections 32 and 34. Described in termsof mathematical functions, the convex sections 28 and 28′ are “concavedown,” whereas the concave section 30 is “concave up.”

As described above, the alternating convex sections 28, 28′ and concavesection 30 act to compensate for the magnetron effects, therebypreventing ions 20 from accumulating a change in cyclotron frequency.That is, the net magnetron effect will be zero, or nearly so, meaningthat the change in cyclotron frequency will also be zero, or nearly so.For example, in the embodiment shown and described herein, thecompensation occurs as follows: the magnetron effects on an ion 20initially induce an increase in cyclotron frequency as the ion 20traverses the first convex section 28 (e.g., from wing 27), then thecyclotron frequency is decreased in the concave section 30. Thecyclotron frequency of the ion 20 is then increased again as the ion 20encounters the second convex section 28′, resulting in a final cyclotronfrequency that is substantially the same as the original cyclotronfrequency of ion 20 before it entered the alternating convex sections28, 28′ and concave section 30. In other words, the combined convexsections 28 and 28′ cancel out or compensate for the cyclotron frequencychange that occurs in the concave section 30. Accordingly, ion 20experiences a net magnetron effect that is zero (or substantially equalto zero) each time it fully crosses or traverses the convex section 28and concave section 30 of the field potential 26 of trapping electricfield 24.

Viewed another way, the convex section 28 and concave section 30 haveopposite effects on the cyclotron frequency such that the coherence orphase of the ions 20 throughout the cell 18 are nearly the same. Whilethere is some slight phase shift in the convex section 28 as thecyclotron frequency is increased and a slight phase shift in the concavesection 30 as the cyclotron frequency is decreased, the phase of theions 20 in the wings 27 and 29 is constant, or nearly so. Therefore,detecting the ions 20 only with the plates in the electrode modules 54in the wings 27 and 29 provides the best peak resolution. If detectionin the convex section 28 and concave section 30 is included, then therewill be some broadening of the peak shape. If the electrostaticcomponents of the convex section 28 and concave section 30 arewell-matched (or nearly so), the overall magnetron effect will bereduced or eliminated (i.e., the magnetron effect will be compensated),with the effect that the observed cyclotron frequency will be the truecyclotron frequency of the ions 20 in a magnetic field of givenstrength. In addition, the phase coherence of the ions 20 will be thesame, or nearly so.

As already mentioned, a wide range of trapping electric fields 24 may beutilized that will result in such compensated magnetron effects andshould be regarded as within the scope of the present invention. Oneclass of electric fields that will result in such compensated magnetroneffects are those that follow the n^(th) root power law (i.e., 10^(n√)).For example, the trapping electric field 24 illustrated in FIGS. 6 a and6 b follows a second root power law (i.e., 10^(1/2)). In anotherembodiment, to be discussed below and illustrated in FIG. 11, thetrapping electric field 124 follows a third root power law (i.e.,10^(1/3)).

The compensation of the magnetron effects is illustrated in FIG. 7,which depicts a computer simulation of a spiral path 48 followed by anexemplary ion 20 (see FIG. 1) during the excitation period. The computersimulation depicted in FIG. 7 was produced by the SIMION® computerprogram referenced above for a trapping electric field 24 substantiallyfollowing the second root power law and for an excitation frequency thatwas matched to the mass-to-charge ratio of an ion 20 having a mass of10,000 atomic mass units (amu). The spiral path 48 depicted in FIG. 7represents the increasing cyclotron radius of the ion 20 as its energyincreases during the excitation process. The ion 20 also follows agenerally reciprocating axial path (i.e., generally back and forth alongthe z-axis of vacuum chamber 12, between wings 27 and 29 of fieldpotential 26) that is induced by the trapping electric field 24.

During this reciprocation, the resonating ion 20 repeatedly traversesthe convex section 28 and concave section 30 of the field potential 26,as illustrated in FIG. 6 b. Because the net induced magnetron effectsare zero (or substantially equal to zero), the cyclotron frequency ofthe resonating ion 20 remains unchanged (or nearly so). This fact isillustrated in FIG. 7 by the substantially straight line 50. The line 50connects the locus of points that correspond to the position of theexemplary resonating ion 20 at intervals of the time period for thecyclotron frequency of the ion 20. Because the points occur at the sameangular position on the spiral path 48, that is, because line 50 issubstantially straight (as opposed to curved), the resonating ion 20(see FIG. 1) depicted in the computer simulation illustrated in FIG. 7is resonating at the true cyclotron frequency for the ion 20.

Another consequence of the compensated trapping electric field 24 isthat ions 20 having the same mass-to-charge ratio will be excited to thesame radius throughout the cell 18, which leads to improved massaccuracy, resolution, sensitivity, and quantitativeness of ionabundances (i.e., number of ions 20 in cell 18). This effect isillustrated in FIG. 8, which depicts a computer simulation of radialexcitation paths 88, 88′ and 88″ followed by three ions having identicalmass-to-charge ratios. The time for excitation of the three ions was thesame. The computer simulation depicted in FIG. 8 was produced by theSIMION® computer program referenced above for a trapping electric field24 substantially following the second root power law and amass-to-charge ratio of about 100.

In addition, the compensated trapping electric field 24 allows ions 20of different types to be excited to the same radii, thereby allowing theobserved signals (e.g., processed data 44) to more accurately reflectthe number of ions 20 within the cell 18, along with improved massaccuracy, resolution, and sensitivity. This effect is illustrated inFIG. 9, which depicts a computer simulation of radial excitationdistances 90, 90′ for ions 20 having different mass-to-charge ratios.Again, the time for excitation of the ions was the same. The computersimulation illustrated in FIG. 9 was produced by the SIMION® computerprogram for a trapping electric field 24 substantially following thesecond root power law. The radial excitation distances 90 are those forions 20 having mass-to-charge ratios of about 100, whereas the radialexcitation distances 90′ are those for ions 20 having mass-to-chargeratios of about 5000.

As mentioned above, the opposed electrode pairs 16 are utilized toproduce or generate the electric fields used in the trapping andexcitation modes. The electrode pairs 16 are also used to detect thedecay of the resonating ions 20. In one embodiment, controller 23 isprovided to allow the various voltage potentials to be applied to theopposed electrode pairs 16 as well as to allow for the detection of thesignals produced by the decay of the resonating ions 20. FIGS. 10 a and10 b are schematic representations of one embodiment of controller 23(see FIG. 1) for providing this functionality.

Referring specifically now to FIGS. 10 a and 10 b, the various pairs ofopposed pairs of electrodes 16, which are schematically illustrated inFIG. 10 b, may be electrically connected to the circuit illustrated inFIG. 10 a. Each of the opposed pairs of electrodes 16 is numbered inaccordance with the convention illustrated in FIGS. 10 b and 11 and isconnected to the corresponding circuit nodes illustrated in FIG. 10 a.For example, the middle or “center” pair of electrodes, numbered 1 and 2in FIGS. 10 b and 11 is connected to various circuit nodes of thecircuit illustrated in FIG. 10 a. The electrodes 16 on the top portionof FIG. 10 b are numbered with odd numbers 3, 5, 7, 9, 11, and 13, withthe suffixes “A” and “B” denoting corresponding electrodes on theopposite sides of the top center electrode 1. Similarly, the electrodes16 on the bottom portion of FIG. 10 b are numbered with even numbers 4,6, 8, 10, and 12, with the suffixes “A” and “B” denoting thecorresponding electrodes on the opposite sides of the center bottomelectrode 2. Electrodes 13A, B, C, and D may be utilized as “gate”electrodes. As will be explained in greater detail below, the potentialson gate electrodes 13 may be set so as to allow ions 20 from an ionsource (not shown) to enter the cell 18. Of course, such gate electrodes13 need not be used if a separate ion gate is provided, or if the ions20 are to be generated or produced within the cell 18.

Referring now primarily to FIG. 10 a, various voltages at various nodesare connected to voltage source 22 illustrated in FIG. 1. During thedetection mode of operation, RF voltages are electrically connected todetector 36 (FIG. 1) to allow for the detection of signals induced onthe electrodes by the resonating ions 20. In the embodiment illustratedin FIGS. 10 a and 10 b, eleven (11) opposed electrode pairs 16 (i.e., 1,3A,B, 5A,B, 9A,B, 11A,B) are used to produce the trapping electricfields 24 (FIGS. 5 a, 5 b), 124 (FIG. 11) whereas nine (9) opposedelectrode pairs 16 (i.e., electrodes numbered 2, 4A,B, 6A,B, 8A,B,10A,B, 12A,B) are used to apply the excitation (e.g., RF) electricfield. These same nine (9) pairs of electrodes are also used to detectthe resonating ions 20 during the detection mode of operation.Alternatively, a greater or fewer number of electrode pairs 16 could beutilized in any given application, as would become apparent to personshaving ordinary skill in the art after having become familiar with theteachings provided herein. Accordingly, the present invention should notbe regarded as limited to embodiments wherein any particular number ofelectrodes are used in the preconditioning, trapping, excitation, anddetection modes of operation.

The remaining two pairs of electrodes (i.e., electrodes numbered 13A-D)may be used to precondition and/or “gate” ions 20 into cell 18. In thisregard, it should be noted that the remaining two pairs of electrodes13A-D need not comprise two opposed pairs but could comprise othernumbers depending on the particular application. For example, only oneor two pairs of electrodes 13 may be required if they are used as akinetic energy filter, while additional pairs may be required to performdipolar “axialization” or some other function that may include the useof both DC and AC electric fields. It is important that, when the ions20 are introduced into the cell 18, they have energies similar topotentials as “high” as possible on wings 27 and 29 of field potential26 (FIG. 6 b) (i.e., at positions near the outermost portions of wings27, 29). If the ions 20 have excessive energies (i.e., greater than atthe “high” portions of the wings 27, 29), they will traverse the cell 18once and exit the far side. If ions 20 with low energies (i.e., similarto that in the concave section 30) are allowed to enter cell 18, theywill remain in the concave section 30 and will have an uncompensatedmagnetron effect.

Still referring to FIG. 10 a, resistors 92, 93, 94, 95, and 96 form avoltage divider between a DC voltage and ground (which may be designatedby the symbol “V”). The values of the various resistors are selected toapply a voltage to the various electrode pairs 16 so that the resultingtrapping electric field 24 has a field potential 26 that will compensatefor magnetron effect. In the example illustrated in FIG. 10 a, thevalues of the resistors 92, 93, 94, 95, and 96 (e.g., having values of39, 12, 15, 23, and 100 kilohms (kΩ), respectively) will result in atrapping electric field 124 having a field potential that follows thethird root power law, as illustrated in FIG. 11. Resistors 97 (e.g.,having values of 10 megohms (MΩ)) couple the DC voltage to the variouselectrodes. Capacitors 98 (e.g., having values of 100 picofarads (pF))decouple the DC bias and are basically radio-frequency shorts to ground.Capacitors 99 (e.g., having values of 1000 pF) couple the RF (AC)voltage to the electrodes. The combined RC time constant for capacitors99 and resistors 97 has a cut-off low-pass frequency that is higher thanthe RF frequencies of interest.

Electrodes 11A,B and 12A,B may be connected to a DC voltage designatedV_(trap). The voltage V_(trap) does not couple RF since it is desired toswitch this signal on rapidly (i.e., not slowed by the RC network). Thevoltage V_(trap) may be set somewhat higher than the voltage onelectrodes 9A,B and 10A,B to filter ions 20 based on kinetic energy. Forexample, only ions above the DC voltage on electrodes 9A,B and 10A,B andbelow V_(trap) will remain in the cell 18 to ensure that the ions 20will oscillate through the middle portion 86 of trapping electric field24 (see FIG. 6 a). Gate electrodes 13A-D may be configured with voltagesV13 _(a,b,c,d) to further filter based on ion kinetic energy or may beconfigured for other pretrapping or preconditioning of ions 20. VoltagesV13 _(a,b,c,d) may be the same or different, depending on the particularfunction desired, as would become apparent to persons having ordinaryskill in the art after having become familiar with the teachingsprovided herein.

Referring back now to FIG. 1, the spectrometer 10 may also be providedwith a detector 36 that is electrically connected to at least some ofthe opposed electrode pairs 16. For example, in the embodimentillustrated in FIGS. 10 a, 10 b, and 11, electrodes 1, 2, 3A,B, 4A,B,5A,B, 6A,B, 7A,B, 8A,B, 9A,B, and 10A,B may be connected to detector 36via nodes V_(rf+) and V_(rf−). Detector 36 detects electrical signals 38induced in the various opposed electrode pairs 16 by resonating ions 20trapped within cell 18. Detector 36 produces output signals 42 that arerelated to the resonating ions 20.

A data processor 40 operatively associated with detector 36 may processoutput signals 42 produced by detector 36 to produce processed data 44.For example, in one embodiment, data processor 40 may be provided with aFourier transform algorithm suitable for performing a Fourier transformon output signals 42 from the detector 36. Accordingly, the processeddata 44 will comprise Fourier-transformed data. Processed data 44 fromdata processor 40 may thereafter be presented in a suitable display 46or any other device or system that will allow a user to interpret theprocessed data 44.

However, because detector systems, data processing systems, displaysystems, and systems for the generation and amplification of therequired excitation fields of the type that may be utilized inconjunction with the present invention are well known in the art andcould be readily provided by persons having ordinary skill in the artafter having become familiar with the teachings provided herein, theparticular detector, data processing, and display systems that may beutilized in one embodiment will not be described in further detailherein.

The ion cyclotron resonance spectrometer 10 may be operated as followsto perform ion cyclotron resonance spectrometry. As illustrated in FIG.1, vacuum chamber 12 may be positioned within a magnetic field so thatthe magnetic field vector 14 is generally parallel with the z-axis ofvacuum chamber 12. The magnetic field vector 14 should be of sufficientstrength so that, when combined with the trapping electric field 24,ions 20 are confined within the cell 18. In one embodiment, a magneticfield having a strength of about 7 tesla in the region generally withinvacuum chamber 12 will provide acceptable results. Vacuum chamber 12should also be evacuated and provided with ions 20 to be studied.

The ions 20 to be studied may be produced in accordance with any of awide variety of processes. For example, in one embodiment, the ions 20may be produced by a suitable ion source (not shown) positioned outsidethe cell 18 (e.g., to the left of cell 18 in FIG. 10 b). Ions 20 maythen be released or “gated” into cell 18 by placing the appropriatevoltages on electrodes 13A-D in FIGS. 10 b and 11. Alternatively, ions20 may be produced within the vacuum chamber 12 itself, such as, forexample, by using a laser (not shown) to ionize sample materialpreviously provided within vacuum chamber 12. In any event, once ions 20have been introduced within the cell 18 within vacuum chamber 12,voltage source 22 is operated to place a trapping voltage function onthe various opposed electrode pairs 16. In the embodiment shown anddescribed herein, this is accomplished by providing a DC voltage to theV_(dc) node illustrated in FIG. 10 a. The voltage divider network ofresistors 92, 93, 94, 95, and 96 will cause various voltages to beapplied to electrodes 1, 2, 3A,B, 4A,B, 5A,B, 6A,B, 7A,B, 8A,B, 9A,B,and 10A,B to be such that the trapping electric field 124 in accordancewith the third root power law (FIG. 11) will be created in the cell 18.

The voltage source 22 then may be operated in an excitation mode inorder to excite the ions 20 contained within the cell 18. In operatingin the excitation mode, voltage source 22 places an alternating current(e.g., an RF) voltage between nodes V_(rf+) and V_(rf−) in FIG. 10 a,the result of which will be the application of the excitation voltagefunction to the opposed pairs of electrodes 16. The excitation voltagefunction is provided in addition to the trapping voltage function sothat ions 20 can be excited but still remain trapped within the cell 18.Generally speaking, the excitation voltage function will cause analternating electric field (e.g., an RF field) to be established in theregion between the opposed pairs of electrodes 16 that will be effectivein causing ions 20 within the cell 18 to become excited (i.e., gainenergy).

The excitation voltage function may comprise any of a wide range offunctions suitable for exciting the ions 20 of interest. For example, inone embodiment, the excitation voltage function may be selected so as tocause the alternating electric field to vary at a fixed frequency, suchas, for example, the cyclotron frequency of the ions of interest.Alternatively, the excitation voltage function may cause the electricfield to vary at some combination of fixed frequencies. In still anotherembodiment, the excitation voltage function may cause the alternatingelectric field to vary across a predetermined frequency range, alsoknown as a “chirp” function.

When the ions 20 are exposed to the excitation electric field, theyresonate, continually gaining energy, which results in an increase inthe cyclotron radius of the ions 20, as depicted by the spiral path 48illustrated in FIG. 7. As the ions 20 resonate within the cell 18, theyalso follow reciprocating axial paths (i.e., generally back and forthalong the z-axis) within vacuum chamber 12, as best seen in FIG. 8. As aresult, the resonating ions 20 repeatedly traverse the convex section 28and concave section 30 of the field potential 26 (FIG. 6 b). Because thetrapping electric field 24 results in a no-net magnetron effect asresonating ions 20 repeatedly traverse the convex section 28 and concavesection 30, the cyclotron frequency of the resonating ions 20 remainsunchanged, as evidenced by the straightness of line 50 illustrated inFIG. 7. In addition, ions 20 having the same mass-to-charge ratio willbe excited to the same radial distance regardless of their z-axispositions in the cell 18, as best seen in FIG. 8. Ions 20 havingdifferent mass-to-charge ratios also will be excited to the same radii,as depicted in FIG. 9.

After a suitable period of excitation, ions 20 trapped within the cell18 may be detected by detector 36. In one embodiment, detector 36detects electrical signals 38 induced on the various opposed pairs ofelectrodes 16 by the movement of the resonating ions 20 via nodesV_(rf+) and V_(rf−) in FIG. 10 a. The data processor 40 may be used toprocess output signals 42 from detector 36 to produce processed data 44.By way of example, in an embodiment wherein the data processor 40 isprovided with a Fourier transform algorithm, processed data 44 maycomprise Fourier-transformed data. The Fourier-transformed data may thenbe presented on display 46.

Various embodiments of ion cyclotron spectrometers according to theteachings provided herein may be used to advantage to perform othertypes of ion detection, excitation, and manipulation processes. Forexample, while the foregoing description of the methods and apparatus ofthe present invention involve the placement of identical DC potentialson the various upper and lower electrodes 58 and 60, this need not bethe case. For example, in another embodiment, a DC offset may be placedbetween the upper and lower electrodes 58 and 60 of each electrodemodule 54. Placing a DC offset on the electrodes 58 and 60 could causethe ions 20 to gradually move laterally to the sides of the cell 18,i.e., toward either of the fins 68 of the field termination unit 56. Theshift direction would depend on the direction of the magnetic fieldvector 14 and the polarity of the DC offset on the electrodes 58 and 60.If the fins 68 are electrically isolated from the outer ring of fieldtermination unit 56, then the fins 68 could be used to detect thelateral displacement of the ions 20.

More specifically, in one such embodiment, each fin 68 would beresistively connected to the outer ring of field termination unit 56 sothat its DC potential would remain the same as that of the outer ring offield termination unit 56. Some or all of the fins 68 on the left sideof the cell 18 would be coupled together (e.g., via a capacitorconnected between adjacent fins 68). Similarly, some or all of the fins68 on the right side of the cell 18 could be coupled together (e.g., viaa capacitor connected between adjacent fins 68). The left and rightgroups of fins 68 could then be monitored for the AC signal created whenions 20 of the shifting (i.e., laterally moving) ion “cloud” induce asignal on either the left or right group of fins 68.

The circuit illustrated in FIG. 10 a could be easily modified to allowan adjustable DC offset to be impressed between the upper and lowerelectrodes 58 and 60. Of course, the DC offset would be applied inaddition to the common DC potential and any AC excitation potential. Anyinadvertent or undesired magnetron motion could be balanced out byadjusting the DC offset to prevent the ion cloud from gradually shiftingleft or right within the cell 18. Balance could be confirmed when nosignal was observed on either group of fins 68. That is, the fins 68could be used to monitor cell performance.

The embodiment described above can also be used in conjunction with themethod described below to provide an alternative way to selectivelydetect ions 20 within the cell 18. For example, and as was describedabove, detection of ions 20 within cell 18 may be performed viadetection of the cyclotron motion. An alternative method would involvethe use of a single excitation frequency to “spin-up” ions 20 of adesired or selected mass-to-charge ratio. The radii of the selected ions20 would increase to the point where they were well beyond the outsideof the remaining ion cloud. At this point, the excitation phase could beterminated. A slight differential potential (i.e., DC offset) could beplaced across opposed electrodes 58 and 60 to cause all of the ions 20to gradually shift toward the left or right group of fins 68. As theexcited ions 20 impact the fins 68, their signal would be detected(e.g., by detector 36). If the DC offset is maintained, the remainingions 20 in the smaller ion cloud would then impact the fins 68, creatinga second detection peak. The intensity of the signal resulting from theimpact of the excited ions could then be compared with the normal FTMSdetection for the selected mass-to-charge ratio.

The method could be extended to detect multiple ion types in the sameion cloud. This would require that the excited ions 20 be detected andthen the DC offset be immediately reversed for a period of time tore-center the ion cloud within the cell 18. Then, another excitationfrequency would be imposed to selectively excite ions 20 of anothermass-to-charge ratio. These excited ions 20 would then be detected in asimilar manner. The process can be repeated so long as sufficient ions20 remain in the cell 18.

Yet another method for multiple ion detection involves ion excitationvia a fixed number of frequencies designed to only excite selectedgroups of ions 20 with desired mass-to-charge ratios. In this method,the amplitude and/or duration of each selected excitation frequencypotential would be selected to excite each group of ions 20 to differingand non-overlapping cyclotron orbit radii. Thus, when the DC offsetbetween the upper and lower electrodes 58 and 60 is used to shift theexcited ions 20 laterally out of the cell 18 and into the fins 68, eachexcited mass-to-charge ratio ion group could create a separate anddistinguishable peak.

Having herein set forth preferred embodiments of the present invention,it is anticipated that suitable modifications can be made thereto whichwill nonetheless remain within the scope of the invention. Therefore,the invention shall only be construed in accordance with the followingclaims.

What is claimed is:
 1. An ion cyclotron spectrometer, comprising: avacuum chamber extending at least along a z-axis; means for producing amagnetic field within the vacuum chamber so that a magnetic field vectoris generally parallel to the z-axis; and means for producing a trappingelectric field within the vacuum chamber, the trapping electric fieldcomprising a field potential that, when taken in cross-section along thez-axis, includes at least one section that is concave down and at leastone section that is concave up so that ions traversing the fieldpotential experience a net magnetron effect on a cyclotron frequency ofthe ions that is substantially equal to zero.
 2. The ion cyclotronspectrometer of claim 1, wherein the field potential includes a firstconcave down section on a first side of the at least one concave upsection and a second concave down section on a second side of the atleast one concave up section.
 3. The ion cyclotron spectrometer of claim1, wherein the means for producing a trapping electric field comprises:a plurality of opposed electrode pairs positioned within the vacuumchamber, the plurality of opposed electrode pairs extending along thez-axis; and a voltage source electrically connected to the plurality ofopposed electrode pairs, the voltage source applying at least a trappingvoltage function to the plurality of opposed electrode pairs to form thetrapping electric field.
 4. A method for performing ion cyclotronspectrometry, comprising: providing ions within a vacuum chamber;producing a magnetic field within the vacuum chamber so that a magneticfield vector is generally parallel to a z-axis of the vacuum chamber;producing a trapping electric field within the vacuum chamber, thetrapping electric field comprising a field potential that, when taken incross-section along the z-axis, includes at least one section that isconcave down and at least one section that is concave up so that ionstraversing the field potential experience a net magnetron effect on acyclotron frequency of the ions that is substantially equal to zero;exciting ions trapped by the magnetic and trapping electric fields; anddetecting excited ions.
 5. The method of claim 4, wherein exciting ionscomprises producing a substantially uniform AC electric field within thevacuum chamber, the substantially uniform AC electric field comprising aplurality of potential lines that are substantially parallel within aregion that is substantially co-extensive with the trapping electricfield.
 6. Ion cyclotron spectrometry apparatus, comprising: a vacuumchamber that extends at least along a z-axis; a plurality of opposedelectrode pairs positioned within the vacuum chamber, the plurality ofopposed electrode pairs extending along the z-axis; means for producinga magnetic field within the vacuum chamber and between the plurality ofopposed electrode pairs so that a magnetic field vector between theplurality of opposed electrode pairs is generally parallel to thez-axis; and a voltage source electrically connected to the plurality ofopposed electrode pairs, the voltage source applying at least a trappingvoltage function to the plurality of opposed electrode pairs, thetrapping voltage function causing a trapping electric field to beestablished between the plurality of opposed electrode pairs, thetrapping electric field comprising a field potential that, when taken incross-section along the z-axis, includes at least one section having aconcave curvature and at least one section having a convex curvature sothat ions traversing the field potential having the concave and convexcurvatures experience a net magnetron effect on a cyclotron frequency ofthe ions that is substantially equal to zero.
 7. The apparatus of claim6, wherein the field potential taken in cross-section along the z-axiscomprises a first wing section and a second wing section on oppositesides of the at least one concave curvature and the at least one convexcurvature section.
 8. The apparatus of claim 7, wherein the first andsecond wing sections are substantially linear.
 9. The apparatus of claim6, wherein the trapping electric field follows a curvature described byan n^(th) root power of
 10. 10. The apparatus of claim 6, wherein thetrapping electric field follows a curvature described by a second rootpower law.
 11. The apparatus of claim 6, wherein the trapping electricfield follows a curvature described by a third root power law.
 12. Anelectrode module, comprising: a generally ring-shaped field terminationunit defining an interior region therein; a first electrode mountedwithin the interior region of the field termination unit; and a secondelectrode mounted within the interior region of the field terminationunit so that the first and second electrodes are positioned in generallyparallel, spaced-apart relation, a combination of the field terminationunit and the first and second electrodes being such that a voltagepotential placed between the first and second electrodes will result ina formation of an electric field having potential lines that aresubstantially parallel throughout a region defined between the first andsecond electrodes.
 13. The electrode module of claim 12, wherein thefirst electrode comprises a pair of inwardly turned ends and the secondelectrode comprises a pair of inwardly turned ends.
 14. The electrodemodule of claim 13, wherein the field termination unit comprises a pairof inwardly extending fins, the pair of inwardly extending fins beingpositioned in generally opposed relation to one another at positionssubstantially midway between the region defined between the first andsecond electrodes.